1. Field of the Invention
The present invention relates to a slot machine for forming a winning line by selecting predetermined symbols from symbol trains arranged on X reels or from X symbol trains stored in a memory, based on a random number table.
2. Related Background Art
In playing a slot machine, a player places a coin or medal into a coin port provided on a slot machine body and manipulates a start lever or the like to simultaneously rotate several reels having a plurality of symbols drawn on outer peripheries thereof. Those several reels have an equal number of symbols arranged on the outer peripheries thereof.
FIG. 1 shows symbol trains arranged on the outer peripheries of the reels in a prior art slot machine. In the prior art slot machine, the symbol trains having an equal number of symbols are drawn on the outer peripheries of all reels. Accordingly, each symbol arranged on the outer peripheries of the reels indicates the stop position. Each symbol train consists of sixteen symbols which correspond to stop positions (16 positions) of the reel. Accordingly, each symbol arranged on the outer peripheries of the reels indicates the stop position. Each symbol train includes four different symbols (7B, 5B, 1B, BL), and winning combinations are (1) 7B--7B--7B, (2) 5B--5B--5B, and (3) 1B--1B--1B. The symbols which constitute the winning combinations are 7B, 5B and 1B. The symbol train on the first reel includes the symbol 7B at a proportion of 2/16, the symbol 5B at a proportion of 2/16 and the symbol 1B at a proportion of 4/16. The symbol train on the second reel includes the symbol 7B at a proportion of 2/16, the symbol 5B at a proportion of 2/16, and the symbol 1B at a proportion of 3/16. The symbol train on the third reel includes the symbol 7B at a proportion of 1/16, the symbol 5B at the proportion of 2/16 and the symbol 1B at the proportion of 3/16. Accordingly when the first, second and third reels are simultaneously rotated, a probability of occurrence of 7B--7B--7B is 4/4096, a probability of occurrence of 5B--5B--5B is 8/4096, and a probability of occurrence of 1B--1B--1B is 36/4096.
The winning or losing is determined by a combination of symbols appeared on a winning line when the reels are stopped. A probability of winning in such a slot machine depends on the types and number of symbols arranged on the outer peripheries of the reels and number of reels. In a recent slot machine, in order to maintain a pay-out rate at a predetermined level and prevent extreme deviation of a probability of winning depending on technique, one random number is selected from a random number table in a given range, and the winning or losing is determined based on the selected random number.
In a three-reel type slot machine, the selected random number is compared with a probability table to determine the type of winning, and the symbols are determined by referencing symbol tables provided for each reel so that the winning condition is met.
In another slot machine, the selected random number is related to the symbols of the reels, and the symbols of the reels are uniquely determined when the random number is selected.
In the prior art slot machine which determines the symbols of the reels by utilizing the random number, the number of random numbers of a random number table and the number of combinations of symbols of the reels are not equal. For example, where 20 symbols are arranged on each of three reels and 8192 random numbers are derived from 13-bit binary numbers, a total number of combinations of symbols generated for the respective reel stop positions is 8000, which is different from the total random numbers for specific symbol combinations. As a result, even if a specific random number is selected with a uniform probability, 2 different random numbers are often assigned to some symbol combinations and the probability becomes substantially nonuniform when the symbols are determined.
In order to solve the above problem, JP-A-62-213782 disclosed a slot machine in which each of X symbol tables to be referenced when symbols are selected from symbol trains to determine the winning has Y Symbols equal to X-order root of 2.sup.n.
In this slot machine, a specific random number is selected from the 2.sup.n random numbers at the probability of 1/2.sup.n and the symbols on the X symbol trains are determined based on the sampled random number. In this case, a probability in determining the symbols by referencing the symbol table is equal to the probability in sampling the random number (1/2.sup.n). Accordingly, the combination of symbols finally provided on the winning line is not biased and non-artificial natural game playing can be attained.
In the above slot machine, it is rare for the X-order root of 2.sup.n to be an integer and the leeway for the selection of the number Y of symbols arranged on the outer peripheries of the reels is narrow. For example, where 8192 random numbers derived from 13-bit binary numbers and two reels are used, the number of symbols in the symbol table is 90.5096 . . . (=(2.sup.13).sup.1/2). Where three reels are used, the number of symbols in the symbol table is 20.1587 . . . (=(2.sup.13).sup.1/3). Where four reels are used, the number of symbols in the symbol table is 9.5136 . . . (=(2.sup.13).sup.1/4). Thus, perfect integers cannot be generated. For example, when three reels are used and 20 symbols in the symbol table is used, a total number of combinations of symbols is 8000 (=20.sup.3), and when 21 symbols are used, it is 9261 (=21.sup.3). Thus, when 20 symbols are used, at least 192 (=8192-8000) random numbers are weighted by probability.
The above disadvantage of the slot machine is common to a mechanically driven slot machine in which reels are mechanically rotated to move the symbol trains and a video type slot machine in which symbol trains are displayed on a CRT screen.